In a nuclear power plant, metal materials such as stainless steel, nickel based alloy or the like are used as structure materials such as a reactor, reactor cooling system or the like. Some elements included in the metal materials become radionuclide by receiving neutrons generated and irradiated from the reactor core. For example, cobalt 58 (58Co) as a radionuclide is generated from nickel 58 (58Ni) by a nuclear reaction shown in equation (1) and cobalt 60 (60Co) as a radionuclide is generated from cobalt 59 (59Co) by a nuclear reaction shown in equation (2).58Ni(n,p)58Co  (1)59Co(n,γ)60Co  (2)
Radio nuclides produced in the reactor core move with the flow of the coolant (water) and then adhere to devices or pipes comprised in the reactor cooling system. The radionuclides adhered to the devices or pipes of the reactor cooling system, in particular, the cobalt 58 or cobalt 60 which irradiates high energy gamma-ray become main radiation source of the radial ray (radiation) received by the worker when the worker performs the plant inspection and so on.
It is important for reducing the exposure dose received by the worker to reduce a concentration of cobalt (Co) which is a coolant flowing in the reactor cooling system or nickel (Ni) which is a parent nuclides thereof. It is effective for reducing nickel concentration or cobalt concentration of the coolant that a method for injecting iron into a coolant and ferritizing and fixing nickel or cobalt, which are contained in the coolant, on the surface of a fuel cladding tube. The reason mentioned above is that the surface of the fuel cladding tube has overwhelmingly large surface area and the ferritization of nickel or cobalt easily proceeds on the surface of the fuel cladding tube by the operation of the boiling and condensation.
For example, radiation exposure reduction techniques proposed as conventional radiation exposure reduction technique are disclosed in following patent documents 1-4.
(1) Japanese Published Unexamined Patent Application (Patent Laid-Open) No. 2000-9889 (JP-A-2000-9889) as patent document 1 disclose the technique for suppressing the radiation dose around a reactor cooling system by injecting iron into a coolant and ferritizing and fixing nickel (Ni) or cobalt (Co), which are contained in the coolant, on the surface of a fuel cladding tube.
(2) Japanese Published Unexamined Patent Application (Patent Laid-Open) No. 5-288893 (JP-A-5-288893) as patent document 2 disclose the technique for suppressing the radiation dose around a reactor cooling system by injecting iron oxide obtained from iron ion into a coolant and ferritizing and fixing Ni or Co, which are contained in the coolant, on the surface of a fuel cladding tube.
(3) Japanese Published Unexamined Patent Application (Patent Laid-Open) No. 7-20277 (JP-A-7-20277) as patent document 3 disclose the technique for suppressing the radiation dose around a reactor cooling system by injecting iron oxide into a coolant and ferritizing and fixing Ni or Co, which are contained in the coolant, on the surface of a fuel cladding tube.
(4) Japanese Published Unexamined Patent Application (Patent Laid-Open) No. 63-229394 (JP-A-63-229394) as patent document 4 disclose the technique for suppressing the radiation dose around a reactor cooling system by reinjecting a metal oxide (crud:chalk river unclassified deposit) filtered at the condensate filter of the reactor cooling system into a reactor core and ferritizing and fixing Ni or Co, which are contained in the coolant, on the surface of a fuel cladding tube.
According to the patent document 1, it is known that iron ions are obtained by the electrolysis operation. The generation speed of the iron ions continuously varies in accordance with a state of a voltage between the electrodes or the surface of the electrodes. Therefore, it is not easy to feed iron to the coolant of the reactor cooling system with uniform flow and difficult to control iron concentration in the coolant. Further, as the electrolysis operation needs mechanical cleaning in order to remove oxide film generated on the surface of the electrodes and so on, a workload for handling the devices used upon the electrolysis operation is heavy.
Meanwhile, as iron oxide has lower reactivity in comparison to iron ion, a good ferritization of the iron oxide can not be expected in comparison to Ni or Co even if the iron oxide injected into the coolant reached at the reactor core. Incidentally, it is not preferable situation for plant operation such that a concentration of iron in the coolant is too high.